A multi-cavity filter has been known as an optical filter having a rectangular bandpass characteristic for allowing light in a desired frequency range to pass therethrough, as described in Non-Patent Document 1. A multi-cavity filter is formed by coupling a plurality of base units (Fabry-Perot etalons) each having a single resonator structure in an optical bandpass filter having a dielectric multilayer structure. In the dielectric multilayer structure, two or more types of dielectric materials having different dielectric constants are layered periodically. In this way, a bandpass characteristic can be obtained in only a particular bandwidth.
More specifically, as shown in FIG. 1, a cavity layer 61 having an optical path length that is a multiple integer of ½ of a given passband wavelength λO (hereinafter simply referred to as a “λO/2”) is sandwiched by reflecting layers 60 so as to form a unit block 20. The reflecting layer 60 includes alternately stacked layers 62 of a high refractive index medium (hereinafter referred to as “high refractive index layers”) and having an optical path length of a ¼ wavelength (hereinafter simply referred to as a “λO/4”) and layers 63 of a low refractive index medium (hereinafter referred to as “low refractive index layers”) and having an optical path length of a ¼ wavelength. Two or more blocks 20 are stacked so as to be symmetrical with respect to each other. Thus, a multi-cavity structure is achieved.
The reflecting layers 60 used for sandwiching the cavity layer 61 are disposed so as to be symmetrical with respect to the cavity layer 61. Each of the reflecting layers 60 includes at least two low refractive index layers and two high refractive index layers.
A reference symbol (L) denotes a low refractive index layer having an optical path length of λO/4. A reference symbol (2L) denotes a low refractive index layer having an optical path length of λO/2. A reference symbol (H) denotes a high refractive index layer having an optical path length of λO/4.
A filter shown in FIG. 1 is a design example of a filter in which the number of unit blocks is 2, and the structure can be represented as (LH)2L2(HL)4(LH)4L2(HL)2. Note that (LH)n represents a layer of a pair of H and L is stacked n times. The representation can be rewritten as LHLH 2L HLHLHLH 2L HLHLHLH 2L HLHL.
The multi-cavity filter described in Non-Patent Document 1 has long been known as a filter for a range from visible light to infrared. Thus, a method for designing the structure of a filter that satisfies demanded characteristics has been established.
In general, the bandpass characteristic of this filter depends on the number of layers in the stacked reflecting layer 60, the thickness of the cavity layer 61, and the number of the unit blocks 20 that are coupled with each other. By controlling these factors, a filter having an optimum characteristic can be designed.
In Non-Patent Document 1, for the multilayer materials, well known Si (index of refraction=3.5) and SiO2 (index of refraction=2.0) are used for a high refractive index layer (H) and a low refractive index layer (L), respectively. The refractive index ratio is 1.75. In this example, twenty-five layers are employed in total. In general, in order to obtain filter characteristics using a sufficient interference effect, more than or equal to twenty layers are employed for such a type of filter.
A multi-cavity filter having such a structure can provide a rectangular bandpass characteristic around a given central wavelength λO, as shown in FIG. 4.23 of Non-Patent Document 1.
On the other hand, a method for manufacturing a terahertz dielectric multilayer periodic structure is described in Patent Document 1 and Non-Patent Document 2.
Patent Document 1 describes a method in which a multilayer film is formed by periodically forming a Si layer and an SiO2 layer using a plasma CVD technique. In the method described in Patent Document 1, since a multilayer film is formed by simply changing a material gas, a layer can be formed at a high speed, as compared with a vapor deposition technique or a sputtering technique.
In addition, Non-Patent Document 2 describes a method in which a periodic structure is formed by polishing a plurality of dielectric substrates or semiconductor substrates to a predetermined thickness and bonding the substrates together.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-109827 [Non-Patent Document 1] “Optical Thin Film User's Handbook”, Shigetaro Ogura, The Nikkan Kogyo Shimbun, 1991, pp. 138-142
[Non-Patent Document 2] Highly tunablephotonic crystal filter for the terahertz range, H. Nemec, P. Kuzel, L. Duvillaret, A. Pashkin, M. Dressel, and M. T. Sebastian, optics letters, vol. 30, No. 5, 2005
However, since a terahertz band lies in a range of wavelength from several hundred μm to several mm, the existing design method and the existing manufacturing method cannot be applied for the following reasons.
First Problem
In existing optical filters, such as the optical filter described in Non-Patent Document 1, used for a range from infrared to visible light, the thickness of a layer is 1 μm or less, and the thickness of the device ranges from 1 to 20 μm. Accordingly, even when the number of stacked devices reaches about 20 to 100, the device can be mass-produced using a vapor deposition technique or a sputtering technique.
However, in order to use a device in a terahertz band from a frequency of 0.1 THz to a frequency of 3 THz, the thickness of a layer becomes as large as about 10 μm to 1 mm, and therefore, the thickness of the device sometimes becomes several mm. Accordingly, if the optical filter is produced using a vapor deposition technique or a sputtering technique, a significant amount of time and costs are required for forming a layer. Thus, it is difficult to mass-produce the optical filter.
To solve this problem, as described in Patent Document 1, layers of Si and SiO2 are periodically formed by using a plasma CVD technique so as to form a multilayer film. However, this optical filter supports only a wavelength range of around a dozen μm. The optical filter cannot cover the entire terahertz band.
Second Problem
In general, a variety of dielectric materials, such as the above-described dielectric oxide material, have a dispersion characteristic in which the dielectric constant varies in accordance with a frequency. As a result of the dispersion, the dielectric constant in a terahertz band markedly varies as compared with that in a range from visible light to near infrared. The above-described optical multilayer filter is designed on the assumption that the optical multilayer filter employs a widely used material having an index of refraction of about 1.5 to 5. However, for example, titanium oxide (TiO2) having an index of refraction of about 2.4 in a visible light range has an index of refraction of about 10 in a terahertz band. That is, the index of refraction increases more than threefold. Since the filter characteristic significantly varies in accordance with the dielectric constant of the material of the filter, desired bandwidth and attenuation gain may not be obtained if an existing design method is used.
Third Problem
When the above-described dielectric oxide material is used in a terahertz band, the dielectric loss tends to increase as a result of the dielectric dispersion, as compared with that in a range from visible light to infrared. As in a microwave band, the Qf constant rule is applicable even in a terahertz band. As the frequency increases, the Q value disadvantageously deteriorates. This is particularly true for a high refractive index material. Accordingly, in an existing design, a sufficient transmission intensity may not be obtained.